1. Technical Field
The present invention relates to an exhaust gas purification method and an exhaust gas purification system, and more particularly to a diesel engine exhaust gas purification method and an exhaust gas purification system which can purify exhaust gas while suppressing deteriorations in drivability and fuel consumption.
2. Background Art
In a diesel engine exhaust gas purification system, an NOx occlusion reduction catalyst is used in order to reduce nitrogen oxides (NOx) contained in exhaust gas, unlike a gasoline engine (see Japanese patent application Kokai publication No. 2009-275561, for example). This is because since the exhaust gas of the diesel engine is a lean atmosphere in terms of the air-fuel ratio, it is impossible to directly employ a three-way catalyst used in a stoichiometric atmosphere as in the case of the gasoline engine.
This diesel engine exhaust gas purification system performs a regeneration operation in which an NOx occlusion material (an alkali metal or alkali earth metal such as K and Ba) is allowed to temporarily occlude NOx within the exhaust gas when it is in the lean state, and the state thereof is periodically changed into rich so as to release the occluded NOx to reduce it by virtue of the three-way function.
In order to change the state of the exhaust gas of the diesel engine into rich, it is necessary to throttle the amount of air-intake, and/or supply fuel into the exhaust gas by post-injection or exhaust injection. However, the former reduction of the amount of air-intake might lead to deterioration in drivability of a vehicle, and the latter fuel supply might lead to deterioration in fuel consumption.